Method and apparatus for the preparation of dichlorosilane
BACKGROUND OF THE INVENTION 
     Dichlorosilane can be prepared from a reaction of FeSi and HCl. However, such a method of preparation produces a low product yield and requires a great deal of distillation to separate the numerous by-products. It therefore has become common practice to conduct a disproportionation of trichlorosilane on suitable catalysts to dichlorosilane and separate any silicon tetrachloride. However, that reaction is slow to attain equilibrium. Most of the catalysts used are secondary and tertiary amines or quaternary ammonium salts (cf. DE-AS 21 62 537). High temperatures and high pressures are used to accelerate achieving equilibrium and to avoid excessively long residence times in the catalyst bed and in the reactor. The use of a high pressure condition presents a high degree of risk because dichlorosilane and any formed H 3  SiCl or SiH 4  are selfigniting. Any leakage can result in extremely violent reactions. In flow-through reactors the content of unreacted trichlorosilane is very high. The trichlorosilane has to be repeatedly redistilled with a high energy consumption before a complete transformation is finally achieved. 
     The problem therefore was to reduce the high cost of energy and especially to reduce the amount of distillation required for the separation of unreacted trichlorosilane. 
     Another problem was to rapidly establish the equilibrium of the disproportionation and to remove the dichlorosilane immediately after its formation. Still another problem was to reduce the cost of the apparatus. 
     SUMMARY OF THE INVENTION 
     The above-stated objects and others can be achieved by the present invention. 
     The present invention is in a method and apparatus for the preparation of dichlorosilane by the disproportionation of trichlorosilane on a solid catalyst bed. The reaction is conducted in a reactor at a temperature of from 10° C. up to the boiling point of the reaction mixture. The method of the invention can be performed at a pressure of from 0.8 to 1.2 bars, but standard pressure is preferred. 
     Dichlorosilane in gas form is removed from the reactor and recovered while fractions of trichlorosilane are condensed and recycled to the reactor. A portion of the liquid reaction phase is taken from the reactor and separated into tetrachlorosilane and into trichlorosilane that is recycled to the reactor. 
     An important feature of the invention is a boiling reaction solution and the prompt removal of dichlorosilane in gas form directly after its formation into a distillation apparatus, trichlorosilane being present in only small amounts in the gas, depending on the temperature during the reaction. The temperature in the reactor should not exceed the boiling point of the reaction mixture, i.e., it is to be no more than 40 to 50° C. The pressure in the reactor and the distillation column is the normal pressure. Air is to be excluded from the reactor and in the distillation column. 
     From the liquid reaction phase which is TCS at the beginning and a mixture of Si Cl 4  and TCS in the equilibrium (depending on the reaction temperature) the tetrachlorosilane formed by the disproportionation has to be separated. Such a separation is achieved by distilling and recycling the trichlorosilane back to the reactor and separating pure tetrachlorosilane from the bulb of the distillation column. 
     In a preferred embodiment, only an amount of fresh tetrachlorosilane corresponding to that which is to be consumed is fed into the reactor. In a highly preferred embodiment of the process, and preferred design of the apparatus, there is only one distillation column. Pure dichlorosilane (boiling point 8° C.) is taken from the top of the column by cooling in a condensor on the top of the distillation column, in this way being separated from TCS due to the difference of the boiling points (TCS boiling temperature 33° C.). Tetrachlorosilane is removed from the bottom bulb of the column. Condensed trichlorisilane is removed from the column at mid-level through a tap and returned to the lower part of the reactor. In the distillation column, the infeed of the gases composed of dichlorosilane and amounts of trichlorosilane, the point of removal of condensed trichlorosilane, and the infeed of the liquid charge of trichlorosilane and tetrachlorosilane, are situated one over the other at mid-level of the column at a suitable distance apart. 
     The reactor has an unheated gas chamber in its upper section for condensation of at least a portion of the trichlorosilane. The lower section of the reactor containing the reactants can be heated by a heating jacket. The reactor also has an overflow for the reactor liquid. 
     The reactor is filled above the sieve plate and up to the level of the liquid with the catalyst bodies which are preferably in the form of balls. The catalyst support can be of numerous materials but preferably consists of SiO 2  made from precipitated SiO 2  by shaping and drying. Preferably the catalyst support has a surface area of 150 to 250 m 2  /cm 3 . 
     An alkylamino trialkoxysilane is preferably chemically bound on the catalyst support by condensation in the presence of water, and the prepared catalyst is then carefully dried. Alkylamino trialkoxysilanes with 1 to 20 carbon atoms in the alkylamino group and 1 to 4 carbon atoms, which can be interrupted by oxygen, in the alkoxy group are preferred. Tertiary amines having 1 to 3 carbon atoms in the alkylene group and 1 to 10 carbon atoms in the alkyl group are preferred in the alkylamino group. 
     The general formula for the catalyst is 
     
         (C.sub.x H.sub.2x+1 O).sub.3 Si(CH.sub.2).sub.z N(C.sub.y H.sub.2y+1).sub.2 
    
     with x=1 to 4, y=1 to 10, and z=1 to 3. 
     These catalysts highly accelerate the disproportionation reaction, even at low temperatures. 
     All traces of moisture are carefully removed from the entire apparatus before the reaction. A protective gas, such as nitrogen, is used during the reaction to prevent access of moisture. 
     The method of the invention is preferably practiced in an apparatus integrated into one unit. The integrated apparatus is in the form of a single distillation column connected to the reactor with the gas feed above the trichlorosilane tap and the infeed of the liquid reactor contents below the point of removal of trichlorosilane. The distillation column is formed of a column in which dichlorosilane is removed at the top and the trichlorosilane is condensed. That distillation column is stacked vertically onto a distillation column for distilling trichlorosilane and removing tetrachlorosilane from the bottom of the still. 
     Further subject matter of the invention is in an apparatus for the practice of the method. The apparatus is formed of a solid-bed reactor with a distillation column in fluid connection therewith, such that the feeding of gases from the reactor into the distillation column is disposed above the point of removal of the trichlorosilane which is recycled to the reactor. This column is disposed above the feed of reactor fluid into the distillation column. Dichlorosilane is removed in a condenser at the top of the column. Tetrachlorosilae is removed from the bottom of the distillation bulb. 
     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects obtained by its use, reference should be had to the accompanying drawings and descriptive matter in which there is illustrated and described a preferred embodiment of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 generally shows apparatus useful for the practice of the method. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     Referring to the Figure, a reactor 1 has an overflow 11 to allow the overflow of liquid from the reactor and a feed 12 whereby fresh and distilled trichlorosilane is fed into reactor 1. Gaseous reaction products are removed from reactor 1 through a gas take-off 13. The reactor contains a bed of solids 10 (shown only in part) which includes the catalyst composition on a sieve plate 9. 
     A distillation column 14 is adjacent to reactor 1 and is in fluid communication there with Distillation column 14 has a distillation bulb or boiler 2, a drive off section 3 and a strengthening section 4. Column 14 also has a product receiver 5, a condenser section 6 and a tap 7 Preferably the distillation column is filled with packings 15 only shown in part. 
     Fresh trichlorosilane is fed into reactor 1 vis line 8 through feed inlet 12. The reactor is heated to boil the charge. The disproportionation reaction takes place in the catalyst bed in reactor 1. The gaseous product formed in the reactor is a mixture of trichlorosilane and dichlorosilane which is removed through the gas take off 13 and led to the strengthening section 4. In the strengthening section 4 DCS is distilled, condensed in the condenser section 6 and collected in the receiver 5; TCS is condensed in the strengthening section 4 and taken off as liquid from tap 7 and led back to the reactor 1 through the feed inlet 12. In the drive off section 3 the mixture of TCS and SiCl 4  is introduced as a liquid by the inlet 16. TCS is distilled off and condensed in the strengthening section 4 and taken off as a liquid from tap 7, while SiCl 4  is condensed in the drive off section 3 and as pure SiCl 4  collected in the boiler 2. 
     A liquid mixture of trichlorosilane and formed SiCl 4  is removed from the reactor by the overflow 13 and is introduced into the drive off section 3 of the column 14 by the inlet 16 just below the take off of liquid trichlorosilane from tap 7 in the middle part of the column. At the beginning SiCl 4  is added to the distillation bulb 2 where the SiCl 4  is heated to ebulliton and is preferably held at its boiling point of about 56° C. In this way SiCl SiC1   4  (boiling temperature 56° C.) distills off TCS in the drive off section of the distillation column. During the distillation condensed SiCl 4  is collected in bulb 2 and separated off by the overflow 17. 
     The progress of the distillation is monitored by temperature control. Trouble-free continuous operation is established when the column temperature directly above the inlet of the liquid reaction mixture from the reactor does not exceed or only slightly exceeds 32° C. 
     The gas room above the liquid level is from 1/3 to 1/8 of the height of the reactor; as this part is cooler, a part of TCS which escaped as a gas was condenses and flows back into the reactor. 
     The catalyst of said general formula on a support, e.g. SiO 2  or ceramic can be used generally for reacting TCS in fluid or gaseous phase to form DCS and SiCl 4  by disproportionation having 1 to 30 g aminosilane fixed on 100 g of the support. For the present process method and the reactor used known catalyst, e.g. SiO 2  or ceramic formed bodies with or without alkoxysilane on the surface can also be used but are less effective. 
     EXAMPLE 1 
     In a nitrogen atmosphere the catalyst composition was placed in the reactor. The catalyst has an alkylamino trialkoxysilane of the general formula 
     
         (CH.sub.3 CH.sub.2 O).sub.3 Si(CH.sub.2).sub.3 N(octyl).sub.2 
    
     which had been fixed on SiO 2  ball material in the presence of ethanol containing water, and had been vacuum dried. 6.2 g of aminosilane per 100 g of catalyst composition was fixed in this manner. The distillation column was also flushed out with nitrogen. 
     SiCl 4  was introduced into the distillation bulb and heated to ebullition. After refluxing began at the top of the column, trichlorosilane was slowly fed into the reactor, and the disproportionating reaction started up. The reactor was heated sufficiently to boil the charge. Due to the forming of gaseous dichlorosilane a mixture of DCS and TCS was led into the distillation column which also began to operate. The temperature at the top and in the middle of the column dropped, while the automatic column top control provided so that only pure dichlorosilane entered into the receiver. 
     Simultaneously formed SiCl 4  containing TCS was withdrawn at the bottom of the reactor through a continuous overflow and led to the drive off section of the distillation column. In this manner per hour and per 100 g of catalyst composition 32.3 g (0.24 mole) of TCS were converted and 11.2 g (0.11 mole) of DCS (purity, by gas chromatography: 97.5% DCS, 1.7% H 3  SiCl, 0.8% TCS) were obtained; furtheron 21.6 g (0.13 mole) of SiC1 4  was separated off as pure reuseable product. When after 1 or 3 hours the equilibrium is reached, i.e. at constant temperatures in the reactor and in all parts of the distillation column just the same amount of fresh TCS is added as is reacted. 
     This result is to be compared with the result obtained by a single passage at 32° C. according to known process conditions under such pressure, that the formed reaction mixture is kept liquid followed by redistillation: The same amount of DCS per 100 g of similar catalyst composition was obtained when 128 g (0.95 mol) of TCS flowed through the same reactor column. This corresponds to four times the amount of TCS, of which 3/4 was unused and had to redistilled. 
     EXAMPLE 2 
     In this Example, a catalyst composition was used which had been obtained by the hydrolysis of an alkylamino trialkoxysilane of the formula 
     
         (CH.sub.3 O).sub.3 Si(CH.sub.2).sub.3 N(C.sub.2 H.sub.5).sub.2 
    
     in the presence of water with the splitting off of methanol followed by careful vacuum drying on a support of SiO 2  balls. The tests on this catalyst were performed in the described apparatus at different reactor heating temperatures. The procedure of example 1 was followed. The results are tabulated below (all data refer to 100 g of catalyst composition): 
     
         ______________________________________                                    
Heating                                                                   
temperature                                                               
           TCS Input   DSC Obtained                                       
                                   SiCl                                   
°C. g/h         g/h         g/h.sup.4                              
______________________________________                                    
25          42.0       14.2        25.5                                   
35          76.7       26.1        49.1                                   
45         100.9       34.9        60.1                                   
50         101.8       35.7        61.6                                   
______________________________________                                    
 
    
     Similarly, (CH 3  O) 3  Si(CH 2 ) 2  N(C 4  H 9 ) 2  can be fixed as catalyst on supports of SiO 2  or ceramic. 
     It will be understood that the specification and examples are illustrative but not limitative of the present invention and that other embodiments within the spirit and scope of the invention will suggest themselves to those skilled in the art.